This invention relates to multilamp photoflash units having circuit means for igniting the flashlamps, and more particularly, to high-voltage photoflash arrays with improved means for providing electrostatic protection.
Numerous multilamp photoflash arrangements with various types of sequencing circuit have been described in the prior art; paticularly, in the past few years. A currently marketed photoflash unit (described in U.S. Pat. Nos. 3,894,226; 3,912,442; 3,935,442; 3,937,946; 3,941,992; 3,952,320 and 4,017,728 and referred to as a flip flash) employs high-voltage type lamps adapted to be ignited sequentially by successively applied high-voltage firing pulses from a source such as a camera-shutter-actuated piezoelectric element. The flip flash unit comprises an elongated planar array of eight high-voltage type flashlamps each having a pair of lead-in wires connected to a printed circuit board by means of eyelets thereon. The circuit board is provided with switching circuitry for causing sequential flashing of the lamps, and an array of respectively associated reflectors are positioned between the lamps and the circuit board. The reflectors for the lamps can be made as a single reflector member shaped to provide multiple individual reflectors for the lamps. The reflector member preferably is electrically conductive, such as being made of metal or metal-coated plastic, and is electrically connected to an electrically "ground" portion of the circuitry on the circuit board; thus, the reflector member functions as an electrical shield reducing the possibility of accidental flashing of the lamps by an electrostatic voltage charge on a person or object touching or near the unit. Such accidental flashing is particularly prone to occur in this instance as the primers used in the high-voltage type flashlamps employed in such arrays are designed to be highly sensitive toward high-voltage discharges. Electrical energies as low as a few microjoules are sufficient to promote ignition of such primers and flashing of the lamps. This high sensitivity is needed in order to provide lamps that will function reliably from the compact and inexpensive piezo-electric sources that are practical for incorporation into modern miniature cameras. Typically, the high voltage pulses provided by the camera are in the order of 500 to 4000 volts.
The use of a conductive reflector unit as an electrostatic shield for the flip flash array is described in the aforementioned U.S. Pat. No. 3,935,442, wherein the reflector is connected to a ground point of the circuit board by means of a conductive U-shaped spring clip which engages a web portion of the reflector unit and extends against a conductive area on the circuit board. This clip-engaged conductive area on the circuit board comprises an enlarged portion of a common circuit conductor which is connected to one lead-in wire of each of the flashlamps. An alternative method of connecting the reflector to a "ground" point of the circuit board is described in the aforementioned U.S. Pat. No. 3,941,992, wherein FIG. 2 thereof shows a wire 77 connected between the metallized reflector and a common circuit run. Both of these reflector grounding methods share a basic deficiency in that initial electrical connection is not always established due to inevitable distortion of the plastic array components during manufacture and assembly. Loss of this mechanical (and electrical) contact renders the lamps of that array subject to unintentional electrostatic flashing.
Another problem found with these grounding methods is that the electrical contact is readily lost under humid conditions, principally because of galvanic corrosion of the thin, aluminized reflector coating at the point of contact.